Unique among the known moons in the solar system, Io is a bizarre world whose properties have only been investigated for the past thirty years. NASA discovered that Io was anything but the cold, dead satellite they expected when the Voyager space probes flew past Jupiter in 1979. The probes showed a scarred and angry moon, bursting with volcanic life as plumes of lava shot far above its surface. Ever since then, Io has been a source of endless fascination for astronomers.
At roughly 3,640 km in diameter, Io is one of the four Galilean satellites orbiting Jupiter (the four moons Galileo Galilei could observe through his telescope: Io, Europa, Ganymede, and Callisto). It is also the closest to the gas giant, approximately 420,000 km away from Jupiter, and mostly made of silicate rock, iron, and some sulfur. The “short” distance from its planet, as well as the gravity of the other Galilean moons, are what keep its volcanic engines running. Gravity pulls on Io like the moon’s gravity pulls on Earth to cause the ocean’s tide, and heat results from the friction that this gravity causes in Io’s interior. The heat leads to a tremendous number of volcanoes, as well as frequent eruptions.
With as many as four hundred active volcanoes on its surface, Io is the most geologically active body in the solar system. The plumes of these volcanoes can shoot hundreds of kilometers into the Ionian sky, thanks to lower gravity and different makeup than most magma on Earth (Io’s magma is mostly composed of basalt, magnesium, and sulfur); the erupted molten rock on Io is believed to be much thinner than ours, about the same consistency as olive oil. Io is believed to have turned itself completely inside out several times, over the billions of years since its formation. On a side note, this constant resurfacing explains why there are hardly any impact craters on its surface, unlike most of Jupiter’s other moons.
Io also has a very thin atmosphere, mostly made of gases and particles ejected during its eruption. Thanks to these particles and a strong magnetic field that Jupiter produces, there is a strong electric current running between Io and Jupiter, which causes giant lightning storms in the gas giant’s atmosphere.
These are just a few of the incredible facts we have recently learned about Io, Jupiter’s hyperactive moon. Not only is it one of the most important discoveries we have made in exploring our cosmological neighborhood, but Io serves as a compelling reminder that the universe is full of surprises, and that we never truly know what we could discover next.